The present invention pertains to communication systems and more particularly to communication systems employing Session Initiation Protocol.
Current trends in telecommunications are moving toward Internet Protocol (IP) related protocols and processes to perform tasks that used to be performed by circuit based technology. One of these internet protocol related protocols is the Session Initiation Protocol (SIP) which is used to set up, tear down and modify generic communication sessions within a communication system. See IETF RFC 2543 on “Session Initiation Protocol”1 (SIP). SIP is a control protocol for creating, modifying and terminating communication sessions with one or more participants. These communication sessions include internet multimedia conferences, internet (or any IP network) telephone calls and multimedia distribution. SIP supports communication session descriptions that allow participants to agree on a set of compatible media types. It also supports user mobility by proxying and redirecting requests to the user's current location.
Although SIP is very generic and flexible, it presents problems in that the set up messages are extremely large relative to over-the-air signaling messages used in many cellular systems. As a result it may take a significant period of time to transfer SIP set up and negotiation messages over slow links such as RF links. The slow transfer time results in a slow set up time for communication sessions.
For example, it takes about one-half second to transfer a SIP invite message over a 9600 BPS link. This results in at least a set up time of one second if both initiating and responding equipments are using RF links. This set up time is unacceptable for many applications such as dispatch, where the total set up time must be very fast.
Several other solutions to the problem employ using standard data compression techniques. These techniques, however, result in reduction of SIP set up messages by less than one-half. These solutions are inadequate to provide a fast set up time for SIP communication systems.
Accordingly, it would be highly advantageous to have a SIP set up message arrangement which substantially reduces the size of SIP set up messages and thereby the time required to set up SIP communications.